Ferrofluid seal apparatuses and ferrofluid sealing methods are widely employed, to provide for both exclusion-type seals and pressure-capacity seals about a rotary shaft. Typically, an exclusion-type ferrofluid seal would comprise an annular permanent magnet and two separate pole pieces, one end of each pole piece extending into a close, noncontacting relationship with the surface of the shaft, to define a pair of radial gaps and to retain a ferrofluid O-ring seal by magnetic flux about the surface to the shaft under each of the radial gaps. Such a dual pole-piece ferrofluid seal apparatus typically has a closed, annular cavity between the respective pole pieces. Where a pressure-capacity-type ferrofluid seal apparatus is desired, a multiple-stage ferrofluid seal apparatus is employed, wherein there are multiple radial gaps, either formed on the ends of the pole pieces or formed on the shaft on the opposite ends of the pole pieces (see, for example, U.S. Pat. No. 3,620,584, issued Nov. 16, 1971).
Dual-stage ferrofluid seal apparatuses have been prepared, wherein the pole pieces are of different thickness, or wherein the radial gap between each end of the pole pieces has been varied, in order to provide for a ferrofluid seal apparatus of extended seal life, such as described in U.S. Pat. No. 4,357,021, issued Nov. 2, 1982. This patent describes a ferrofluid rotary-shaft seal apparatus of extended seal life, wherein one of the pole pieces has a varying radial-gap width caused by a tapered end on one pole piece, so that in operation the ferrofluid under the thinner pole piece evaporates first and forms an air gap into the air cavity between the pole pieces, while the ferrofluid, retained at the wider, tapered gap width, provides for extended ferrofluid exclusion seal life.
Also, ferrofluid rotary-shaft seal apparatuses of extended seal life have been described for use as an exclusion seal, particularly with computer-disc-drive spindles, wherein the seal apparatus comprises an annular permanent magnet and a pair of pole pieces, with the pole pieces generally of L-shaped configuration, to form a single ferrofluid O-ring seal about the surface of the shaft, as described, for example, in U.S. Pat. No. 4,357,024, issued Nov. 2, 1982. This patent describes L-shaped pole pieces of the same or different thicknesses, and wherein each of the pole pieces forms a uniform radial gap with the surface of the shaft element, and wherein extensions of the pole pieces extend toward each other, to form an interstage gap, so that the ferrofluid is retained within the interstage and the radial gaps, to form a single ferrofluid O-ring seal apparatus about the surface of the shaft element. The patent describes a single-stage ferrofluid exclusion-type seal, wherein the radial gaps between the ends of the L-shaped pole pieces are uniform; however, the interstage gap between the respective pole pieces may vary, in order to form the desired generally sectional, T-shaped ferrofluid seal.
It is desirable to provide for a ferrofluid exclusion or pressure-type seal which would provide exclusion or pressure protection between a first and second environment separated by the ferrofluid O-ring seal, and wherein any change in gas pressure in the respective environments may be transferred between such environments or to another or third region.